1. Field of the Invention
This invention relates to a superconductive magnet apparatus for use in a medical magnetic resonance imaging device and the like.
2. Description of Prior Art
FIG. 5 is a schematic sectional view of a prior art superconductive magnet apparatus and FIG. 6 is a circuit diagram of the superconductive magnet apparatus of FIG. 5.
In these figures, reference numeral 1 represents a helium container in which liquid helium 2 is stored, 3 a superconductive coil which is immersed in the liquid helium 2 and stored in the helium container 1, and 4 and 5 radiation heat shields arranged to double-enclose the helium container 1.
Reference numeral 6 represents a vacuum container arranged to enclose the radiation heat shield 5 and contain the helium container 1 and the radiation heat shields 4 and 5 in a vacuum atmosphere. Numeral 7 denotes a refrigerator for cooling the radiation heat shields 4 and 5. The helium container 1, the radiation heat shields 4 and 5, and the vacuum container 6 are thermally insulated from one another.
As shown in FIG. 6, to the superconductive coil 3 are connected a protective element 8 and a permanent current switch 9 (to be abbreviated as PCS hereinafter) in parallel to each other which are coupled to a power source 11 for magnetization and demagnetization provided outside the vacuum container 6 by a power supply lead 10.
To operate the superconductive magnet apparatus constructed as described above, the power supply lead 10 is first attached and then a current is supplied from the power source 11 for magnetization and demagnetization to carry out magnetization. Thereafter, when a predetermined magnetic field output is achieved, the PCS 9 is turned on to keep the current in the superconductive coil 3 (permanent current mode). Thereby, a high and stable magnetic field particular to the superconductive magnet apparatus can be obtained. Thereafter, to minimize the penetration of heat, the power supply lead 10 is removed to complete magnetization.
Meanwhile, for demagnetization, the power supply lead 10 is attached and a current for magnetization is supplied by the power source 11 for magnetization and demagnetization to turn off the PCS 9. The current is then made null by the power source 11 for demagnetization and the power supply lead 10 is removed to complete demagnetization.
Since the superconductive magnet apparatus of the prior art is constructed as described above, when a quench phenomenon occurs in the superconductive coil 3 for some reason or another, the liquid helium contained in the helium container 1 is consumed in large quantity because the superconductive coil 3 is arranged isolatedly within the helium container 1, almost all the energy of the superconductive coil 3 is changed into Joule heat by resistance generated by the quench, and the temperature of the superconductive coil 3 is elevated by the heat.